1. Field of the Invention
The present invention relates to the preparation of optionally asymmetric acetals from vinyl ethers and alcohols, using easily removable catalysts. More specifically, the present invention relates to the synthesis of polymerizable photoreactive compounds in reaction mixtures comprising high concentrations of vinyl ethers.
2. Description of the Related Art
Radiation curable compositions for food packaging require the use of low migrating photoreactive compounds. In digital printing techniques, such as ink jet, polymerizable photoreactive compounds are preferred for reasons of low viscosity. For both ecological and economical reasons, this requires an efficient synthesis of these polymerizable photoreactive compounds, wherein the solvent use, the number of steps and the number of isolations of both intermediates and end products in synthetic processes are minimized.
WO 2009/053305 (AGFA) discloses vinyl ether (meth)acylates, such as 2-(vinylethoxy)ethyl acrylate, as preferred monomers for the design of low migration ink jet inks.
The presence of the vinyl ether opens up the possibility to use the monomer of the ink carrier directly as reagent for the synthesis of polymerizable photoreactive compounds, without the need for isolating the polymerizable photoreactive compounds, as disclosed in WO 2010/069758 (AGFA).
This approach requires a highly efficient catalyst for the synthesis of asymmetric acetals from alcohol functionalized photoreactive compounds and vinyl ether containing monomers in the presence of high concentrations of these monomers. However, synthesis at elevated temperatures in highly concentrated vinylether monomer solutions can lead to spontaneous unwanted polymerization of the monomers, imposing considerable safety risks due to high exothermic polymerization leading to a thermal runaway.
For reasons of shelf life stability, the catalyst has to be removed from the end product to avoid degradation and side reactions upon storage. Furthermore for food packaging applications, the residual catalyst has also to be removed for migration reasons.
Typical catalysts used for the preparation of asymmetric acetals by addition of an alcohol to alkenyl-ethers, such as vinyl ethers, are protic acids with a sufficient low pKa, such as hydrochloric acid, phosphoric acid, sulfonic acids, sulfuric acid, and carboxylic acids substituted with electron withdrawing groups such as fluorine and chlorine. Other preferred type of catalysts are organic salts of sulfonic acids, such as pyridine salts.
The use of hydrochloric acid has been disclosed in several documents (e.g. Trofimov et al., Tetrahedron Letters, 49, 3104-3107 (2008)).
The use of phosphoric acid has been disclosed by Toshiaki et al. (Tetrahedron Letters, 47, 3251-3255 (2006)).
The use of sulfonic acids as catalyst has been disclosed in numerous documents (e.g. Munro et al., Bioorganic and Medicinal chemistry, 16(3), 1279-1286 (2008); Snowden et al. Helvetica Chimica Acta, 89(12), 3071-3086 (2006), Lucatelli et al., Journal of Organic Chemistry, 67(26), 9468-9470 (2002); Wipf et al., Tetrahedron Letters, 40(28), 5139-5142 (1999)) Typical examples are p.-toluene sulfonic acid, 10-camphor sulfonic acid and methane sulfonic acid.
The use of sulfuric acid has been described by Rappe et al. (Justus Liebigs Annalen der Chemie, 601, 84-111 (1956)).
The use of carboxylic acids substituted with electron withdrawing substituents has been disclosed in a number of documents (e.g. Rivillo et al., Angewandte Chemie, International Edition, 46(38), 7247-72450 (2007); WO 2007/010483 (FIRMENICH); Alvarez de Cienfuego et al., Tetrahedron: asymmetry, 17(2), 1863-1866 (2006); US 2005171062 (ALLERGAN INC). Typical examples are trifluoroacetic acid and trichloroacetic acid.
The use organic salts of sulfonic acids has been disclosed in several documents (Lee et al. Bulletin of the Korean Chemical Society, 28(4), 513-514 (2007); Hattori et al., Organic Letters, 10(5), 717-720 (2008); Nakamura et al., Organic Letters, 10(2), 309-312 (2008); Nicolau et al. Journal of the American chemical Society, 129(48), 14850-14851 (2007); Nakamura et al., Tetrahedron, 63(35), 8670-8676 (2007)). A typical example of an organic salt of a sulfonic acid is pyridinium tosylate.
Occasionally, also Lewis acids have been reported as catalyst (Alper. H., Synthesis 1972, 81).
Several transition metals have also been shown effective as catalyst for the synthesis of asymmetric acetyls from alkenylethers and alcohols (Maity, G; Synth Commun 1993, 23, 1667; Iqbal, J; Synth Commun 1989, 19, 901; Kantam, M; Synth Commun 1993, 23, 2225; Bhuma, V; Synth Commun 1992, 22, 2941; Ma, S; Tetrahedron Lett 1993, 34, 5269; Molnar, A; Tetrahedron Lett 1996, 37, 8597).
Heterogeneous catalysis has been reported frequently (Bongini, A; Synthesis 1979, 618; Johnston, R; Synthesis 1988, 393; Olah, G; Synthesis 1983, 892; Menger, F; J Org Chem 1981, 46, 5044; Hoyer, S; Synthesis 1986, 655; Upadhya, T; Synth Commun 1996, 26, 4539; Campelo, J; Synth Commun 1994, 24, 1345; Bandgar, B; Synth Commun 1995, 25, 2211; Kumar, P; Synthesis 1993, 1069; Chavez, F; Synth Commun 1992, 22, 159; Patney, H; Synth Commun 1991, 21, 2329; Campelo, J; Synth Commun 1992, 22, 2335).
Acetonyl triphenylphoshonium derivatives have also been reported as catalysts for converting alcohols into asymmetric acetals (Hon et al., Tetrahedron, 57, 5991-6001).
In highly concentrated vinyl ether solutions, the use of strong acid catalysts, such as sulfonic acids, lead to cationic polymerizations as side reaction. This not only results in loss of yield of the desired compound but also leads to safety risks due to potential thermal runaways. Therefore, medium acidic catalysts, such as trifluoroacetic acid and pyridinium salts of sulfonic acids, are particularly preferred catalysts for use in highly concentrated vinyl ether solutions.
From a synthetic point of view, both soluble and resin based type of catalyst can be used. Soluble catalysts such as trifluoroacetic acid and trichloroacetic acid are often compatible with a broad scope of reaction circumstances. However, removal of the catalyst can be laborious, generating extra chemical waste and cost.
Resin based catalysts, such as crosslinked polyvinyl pyridinium sulfonates, are often easily removable from reaction mixtures by simple filtration. However, balancing the equivalence of sulfonic acid and pyridine moieties is not always straightforward in a solid crosslinked resin based catalyst. Small amounts of residual sulfonic acids can leach into the reaction mixture, acting as strong acid initiator for unwanted cationic polymerization.
Zwitterionic compounds are hardly being documented in the synthetic literature as catalysts for organic transformations, apart from WO 2012/168458 (ECOSYNTH BVBA), which discloses the use of a broad scope of zwitterionic catalysts in esterification reactions, using classic reaction conditions in solvent medium such as azeotropic removal of water.
Therefore, for both safety reasons and process simplicity there is still a need for a catalyst being intrinsically safe by avoiding potential excess of strong acids, while still being easily removable to avoid high process costs and waste generation.